B737 flight crew reported the aircraft failed to pressurize while accomplishing the pressurization test during climb on a test flight.

Narrative

The purpose of this flight was to validate normal and backup system operations for the 737 nose number XXXX after completion of D check maintenance actions. I was the Flight Test Captain flying from the left seat as SIC for this test flight. During climbout from ZZZ; we were vectored off of the ZZZZZ departure to a heading of 270. I was intermittently hand flying and using autopilot during the climb in order to complete the checks and procedures called for in the 737NG Flight Test Manual. The right seat Flight Test Captain and I were splitting duties to work through the climb phase test procedures; and I had just completed test procedures XX-X (Yaw Damper) and was working through the various autopilot and flight director functions in XX-Y and XX-Z. The right seat Flight Test Captain was working on XX-A (Maximum Delta-P) during this entire time; since it takes a while to complete. Just prior to the depressurization event; I noticed that my ears were being challenged to keep up and that the right seat Flight Test Captain was saying something about the valves not responding to his commands. I was hand-flying at this point and happened to notice the cabin rate of climb was full-scale up; at over 4;000 feet per minute of climb rate. I then realized that cabin delta-pressure was showing 0 psi; and we both proceeded to don our oxygen masks; established crew communication; [requested priority handling] with ATC; and expedited our descent to below 10;000 feet. Once below 10;000 feet; we removed the oxygen masks and recovered normally.